1. Field of the Invention
The present invention relates to a dielectric ceramic composition and a monolithic ceramic capacitor. More particularly, the present invention relates to a dielectric ceramic composition suitable for use in a monolithic ceramic capacitor that is employed in high-temperature environments such as in car-mounted equipment, for example, and to the monolithic ceramic capacitor constituted by using the dielectric ceramic composition.
2. Description of the Related Art
Generally, the high-temperature side limit of the guaranteed temperature range for a monolithic ceramic capacitor is set to about 85° C. for ordinary consumer-oriented products and to about 125° C. for products that are required to have higher reliability. However, insulation and reliability (life characteristic) at higher temperatures, i.e., at about 150 to 175° C., have been recently demanded in environments such as cars, etc.
For that reason, a material containing, e.g., (Ba1-xCax)mTiO3 as a main component, high temperature endurable, and having a good dielectric constant has been employed as a dielectric ceramic composition for such monolithic ceramic capacitor.
Japanese Unexamined Patent Application Publication No. 2005-194138, for example, discloses a dielectric ceramic composition comprising a composition formula of 100(Ba1-xCax)mTiO3+aMnO+bV2O5+cSiO2+dRe2O3 (where Re is at least one metal element selected from among Y, La, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, and a, b, c and d denote values in terms of moles), and which substantially satisfy the following conditions:0.030≦x≦0.20,0.990≦m≦1.030,0.010≦a≦5.0,0.050≦b≦2.5,0.20≦c≦8.0, and0.050≦d≦2.5.
For the dielectric ceramic composition disclosed in the above-cited Japanese Unexamined Patent Application Publication No. 2005-194138, it is confirmed that the dielectric ceramic composition has excellent high-temperature loading reliability, i.e., about 10 hours or longer in terms of mean time to failure (MTTF), when a DC current having the field strength of about 10 V/μm is applied at about 150° C. However, the above-cited Japanese Unexamined Patent Application Publication No. 2005-194138 does not consider the high-temperature loading reliability, for example, when a DC current having the field strength of about 20 V/μm is applied at about 175° C.
When the amount of the added Re is increased in order to improve the reliability in the dielectric ceramic composition disclosed in the above-cited Japanese Unexamined Patent Application Publication No. 2005-194138, the temperature-dependent change rate of the relative dielectric constant is degraded. It is therefore impossible to realize high reliability and a flat temperature-dependent change rate of the relative dielectric constant simultaneously.